Based on the 2001 - 2004 NSBRI research project by
Heiko Hecht, Laurence R. Young, Charles M. Oman, Bernard Cohen, Mingjia Dai, Pau DiZio, James Lackner, William H. Paloski, Fred Mast, Malcolm M. Cohen, Robert B. Welch, Lee Stone
For images, demonstrations and video, click here.
Traditional countermeasures against the adverse effects of prolonged weightlessness, such as exercise, resistive garments and lower-body negative pressure, appear to be insufficient in practice and are often too inconvenient for astronauts. AG represents a potential countermeasure that is unique. It promises salutary effects on bone, muscle, cardiovascular and vestibular function. Rather than alleviating the symptoms, it attempts to remove their cause. Although long a favorite topic of scientists and science fiction authors, it is only now receiving serious attention for space flight experiments and validation (Young, 1999). Several recent task groups and countermeasure workshops conducted by NASA have refocused attention on AG for extended missions. Spacecraft size dictates that any AG centrifuge tested in the foreseeable future be of limited radius (on the order of 1-3 m). The largest diameter human centrifuge being considered for installation on Spacehab, is under 2.5m in diameter, thus permitting a short astronaut only to sit or bicycle, but not to stand up. Centripetal accelerations on the order of 1 g (9.8 m/sec2) at the rim will therefore require relatively high angular velocities (on the order of 30 rpm). At these speeds, AG will create disruptive sensory effects as soon as the astronaut starts to move. Limb movements are deflected and, more importantly, head movements cause unexpected semicircular canal inputs as the result of Coriolis cross-coupling between certain head movements and centrifuge rotation. One might argue that during brief centrifugation on a short-radius centrifuge (SRC) the head is best restrained to eliminate disturbances. Indeed, this is the approach taken by the Nihon University group. However, restraining the head seriously limits exercise, recreation, and comfort in the device. Movement is mandatory during long-term centrifugation (e. g. in a rotating spacecraft) and it is desirable during intermittent centrifugation (e. g. when combined with exercise). Thus, AG for in-flight gravity replacement therapy requires that crewmembers be capable of rapidly adapting to the unexpected canal inputs with minimal side- or after-effects. Furthermore, it will be essential for astronauts to retain the adaptation to the 0-g state in order to avoid "Space Adaptation Syndrome" each time they transition from the centrifuge to weightlessness.
In our ongoing experimental efforts we address some of the most important research questions requiring answers prior to AG implementation for a long mission. The premise of the research is that AG works in principle. See the Proceedings of the 18th Annual Gravitation Physiology Meeting, Copenhagen (special issue of the Journal of Gravitational Physiology, Vol. 4(2), 1997) and the workshop report by Paloski and Young (1999). The early Russian tests centrifuging rats and the limited human self-generated AG on Skylab are both encouraging. However, we know little about how to administer effective AG under spacecraft size and budget limitations (Greenleaf, Haines, Bernauer, Morse, Sandler, Armbruster, Sagan & van Beaumont, 1975; Vil-Viliams, 1994; Iwasaki, Sasaki, Hirayanagi & Yajima, 1998). Promising results have recently been obtained at Nihon University by Yajima, Iwasaki, Ito, Miyamoto, Sasaki & Hirayanagi (2000), who were able to demonstrate the efficacy of brief centrifugation as a countermeasure during extended bedrest. Daily 60 min AG sessions at 2-g were sufficient to prevent cardiovascular deconditioning. Subject cardiovascular training and head movement restrictions were required.
The remarkable ability of the nervous system to adapt to the altered gravity of space flight brings with it the built-in disadvantage of producing motion sickness, sensory illusions, and motor deficiencies when adaptive states are changed (Young, Oman, Watt, Money & Lichtenberg, 1984). We currently investigate if head and body movements during high rate AG are tolerable and how such AG can be implemented most efficiently. We search for methods to minimize the undesirable side-effects of multiple neurovestibular adaptation associated with intermittent AG.
Our (MIT) experiments on the Short-Radius Centrifuge (SRC) encourage the use of a SRC as a viable countermeasure (Hastreiter & Young, 1997; Young, Hecht, Lyne, Sienko, Cheung & Kavelaars, in press). Inappropriate eye movements (vestibulo-ocular reflexes), motion sickness and perceptual illusions are all reduced after several adaptation periods. Short daily exposures to head movements while rotating appear to yield significant adaptation. Additionally, experience with intermittent off-axis rotation on the Neurolab rotator demonstrated tolerance to high rotation rates and centrifugation in space (Moore, Clement, Raphan, Curthoys, Koizuka & B. Cohen, 2000). The Brandeis Slow Rotating Room (SRR) has yielded a wealth of information concerning the process of sensorimotor adaptation to movements in a rotating framework (Lackner & DiZio, 1998. Other experiments at Johnson Space Center (JSC) show important adaptive and maladaptive changes in head and body control following centrifugation (Kaufman, Wood, Gianna, Black & Paloski, 2000).
AG feasibility may be limited by the potential side-effects that accompany adaptation to a rotating environment. We believe that in weightlessness a major sensory conflict disappears because the conflicting gravito-inertial signals on the otolith organs are eliminated. Space experience supporting this belief includes the absence of neurovestibular consequences of cross-coupled head movements in Skylab (Graybiel, Miller & Homick, 1977) and in parabolic flight (Lackner & Graybiel, 1984), the absence of motion sickness or "nystagmus dumping" during post-rotatory head pitch on SLS-1 and SLS-2 (Oman & Balkwill, 1993; Oman, Pouliot & Natapoff, 1996) and in parabolic flight (DiZio & Lackner, 1988), and during short-radius centrifugation on Neurolab (Moore et al, 2000). However, the negative experiences of the IML-1 crew to in-flight rotation advise caution. We currently lack a full understanding of the mechanism and the limits of such adaptation. For instance, we do not know if intermittent or continuous AG works best, and AG has not yet been put to a serious test with humans in a 0-g environment. Since very few studies have investigated adaptation to short-radius, high-rate centrifugation, we seek to extend this knowledge to the particular case of short-radius centrifugation.
Even without AG, extended space travel, such as a mission to Mars, requires substantial sensorimotor adaptation. The astronaut has to be functional in several gravitational environments (1g, 0g, 0.38g). Short-radius centrifugation would introduce yet another - albeit intermittent - gravitational environment. Thus, the ability of the astronaut to change adaptive states quickly is critical to the success of AG. The altered sensory environments often generate disturbing motor-sensory feedback whenever movements are made. If the altered environment is rotating, as on a centrifuge, these sensory effects are complicated by Coriolis forces and inappropriate signals from the semicircular canals (Guedry, 1974; Gillingham & Previc, 1996; Young, 1983).
People adapt to such sensory rearrangement changes, but they normally adapt slowly over the course of several days or even weeks. Short-radius AG as a countermeasure is designed to deal with space missions in a very particular fashion. Our senses and motor system still need to function in 0-g. Thus, the astronaut must adapt to function effectively in two environments, centrifugation and 0-g. This includes exercise and probably recreation during centrifugation. And consequently head and limb movements will have to be made during centrifugation. AG will work only if the sensorimotor system can be functional in different g-environments while requiring very little or no time to switch between adaptive states. Such state changes need to be made smoothly and with minimal adverse effects (e. g. without motion sickness). That is, context-specific adaptation has to be acquired and maintained over longer periods.
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